Method for producing an electroformed heat exchanger



A. A. MARCO Jan. 23, 1968 METHOD FOR PRODUCING AN ELECTROFORMED HEATEXCHANGER 7 Sheets-Sheet 1 Filed Dec. 8, 1964 FIGURE 2 FIGURE 3INVENTOR.

ALEX AMARCO aw f0 {1&5

AGENT Jan. 23, 1968 A. A. MARCO 3,364,548

METHOD FOR PRODUCING AN ELECTROFORMED HEAT EXCHANGER Filed Dec. 8, 1964'7 Sheets-Sheet 2 FIGURE 4 FIGURE 6 INVENTOR.

ALEX A. MARCO BY AGENT A.A. MARCO Jan. 23, 1968 METHOD FOR PRODUCING ANELECTROFORMED HEAT EXCHANGER '7 Sheets-Sheet 5 Filed Dec. 8, 1964 FIGURE7 FIGURE 7A 8 E R U m F m T N E v N ALEX A. MARCO FIGURE 9 A ENT Jan.23, 1968 MARCO 3,364,548

W METHOD FOR PRODUCING AN ELECTROFORMED HEAT EXCHANGER Filed Dec. 8,1964 7 Sheets$heet 4 FIGURE IO FIGURE ll INVENTOR. ALEX A. MARCO A BYZM' Q AGE T A. A. MARCO Jan. 23, 1968 METHOD FOR PRODUCING ANELECTROFORMED HEAT EXCHANGER 7 Sheets-Sheet 5 Filed Dec. 8, 1964 FIGUREI2 FIGURE 13 INVENTOR ALEX A. MARCO BY yaw $45 AGENT A. A. MARCO Jan.23, 1968 METHOD FOR PRODUCING AN ELECTROFORMED HEAT EXCHANGER 7Sheets-Sheet 6 Filed Dec. 8, 1964 FIGURE l4 FIGURE l5 INVENTOR ALEX A.MARCO r m AGEN A. A. MARCO Jan 23, 1968 METHOD FOR PRODUCING ANELECTROFORMED HEAT EXCHANGER '7 Sheets$heet 7 Filed Dec. 8, 1964 FIGURE16 33 FIGURE l7 INVENTOR ALEX A. MARCO BY/ 2/44? W AGENTM United StatesPatent 3,364,548 METHUD FOR PRODUCING AN ELECTRO- FORMED HEAT EXCHANGERAlex A. Marco, Los Angeles, Calif. (410 E. Duar'te Road, Arcadia, Calif.91006) Filed Dec. 8, 1964, Ser. No. 416,836 11 Claims. (Cl. 29157.3v)

ABSTRACT OF THE DISIILGSURE This disclosure concerns a process for thefabrication of articles such as fluid-to-fiuid heat exchangers whichhave at least two separate passages which are in close thermal contact.The method described involves the use of sacrificial tooling which ischemically dissolved after the construction material is deposited bymetal spray or electroplating. The method allows complex multi-chamberconstruction without welding or other heat processes and permits maximumstrength in compact, thin-wall assemblies. The method also eliminatesthe need for expensive jigs and other tooling.

This invention relates to heat exchangers and more particularly to highperformance heat exchangers which have the greatest rate of heatexchange per unit of volume.

Heat exchangers are very extensively used in process industries, inconnection with power generating systems and frequently, in verysophisticated forms, in modern military equipment.

Prior art heat exchangers of the liquid-to-liquid and liquid-to-gastypes (the types with which the present invention is primarilyconcerned) have been constructed in a variety of forms and designs. Theuse of tubing formed into a labyrinth immersed in a vessel is a commonand well known simple type of heat exchanger.

Most high performance liquid-to-liquid heat exchangers are necessarilybuilt on the counter flow principle. That is to say that, given a hotfluid to be cooled and a cooler fluid to absorb the heat, the coolerfluid coolant would begin to come into thermal contact with the heatexchanging elements at a point where the fluid to be cooled is justleaving the heat exchanger. After the coolant is warmed by heatabsorption as the process of cooling the hot fluid progresses, thecoolant contacts the cooling elements carrying the hot fluid atprogressively higher temperature locations until the last contact of thecoolant with the hot fluid to be cooled is made as the hot fluid entersthe heat exchanger and the coolant leaves the heat exchanger. Obviously,if the direction of flow of the coolant and hot fluid through the heatexchanger are in the same direction, the temperature rise of the coolantbecomes a limiting factor in connection with the terminal temperature ofthe hot fluid as it leaves the heat exchanger. Thus the importantconcept of counter flow is applied to the present invention.

In prior art heat exchangers, the effort to maximize the amount of heatexchange surface usually resulted in the evolution of a device which wasboth expensive and difficult to manufacture. For example, where largeamounts of series or parallel tubing are employed, many Welded, brazedor soldered joints become necessary. Not only are these manufacturingprocesses time consuming and therefore costly, but to the extent thatthey involve the application of heat to materials such as copper (whichare so desirable for use in heat exchanger applications because of theirhigh thermal conductivity) these metals are annealed to a condtion ofrelative softness and greatly decreased strength. Thus if a prior artheat exchanger is to be used under conditions of substantial appliedpressure to one or both of the fluid components, this fact must be takeninto consideration in the stress design of the heat exchanger elements.The result usually is greater weight and thickness of material. Thefurther result is therefore an increase of size and weight. Accordingly,the general objective of the present invention was the development of aheat exchanger of a high order of thermal performance in a minimumpackage and at a comparatively low cost without complex tooling.

In evolving the present invention, it was realized that the more nearlythe two fluid components can be brought into perfect thermal contact,the greater will be the overall thermal efiiciency of the exchanger. Itwill readily be realized that if the coolant fluids were constrainedinto thin planer cross sections of flow separated only by thinconductive membranes a greatly improved thermal efliciency should bepossible. In maintaining these separation membranes at minimum thicknessit is therefore exceedingly important that a method of fabrication bedevised which would not result in the annealing of the membranematerial. Heat exchangers made in accordance with the structure andmethod of the present invention have been found to be highly eflicientnot only on an absolute performance basis but also in terms of rate ofheat exchange per unit volume.

In explaining the present invention typical drawings which will beexplained in terms of typical materials are presented.

FIGURE 1 depicts the sheet metal stacking operation which is the firststep of the process.

FIGURE 2 illustrates the completed stack resulting from the process ofFIGURE 1.

FIGURE 3 illustrates the second or aluminium spraying step of theprocess.

FIGURE 4 illustrates the third step of the process namely the machiningof the laminated edges.

FIGURE 5 illustrates the fourth step of the process which comprises anoverall copper plating operation.

FIGURE 6 illustrates the fifth step in the operation which is thewedge-machining of the small end faces.

FIGURE 7 illustrates the masking or sixth step in the process.

FIGURE 8 illustrates the seventh and eighth step of replacing andunmasking. 1

FIGURE 9 illustrates the ninth step, of manifold mandrels.

FIGURE 10 illustrates the tenth step in which the assembly is againplated.

FIGURE 11 is a cutaway of a finished heat exchanger showing flow of hotand cold fluids.

FIGURE 12 is a combination mandrel and expendable jig for a secondembodiment of the present invention.

FIGURE 13 illustrates a stacking or assembly step for a secondembodiment.

FIGURE 14 illustrates the aluminium spraying step for the secondembodiment.

FIGURE 15 shows the machining step for the second embodiment.

FIGURE 16 illustrates the replating step for the second embodiment.

FIGURE 17 depicts a midsection cutaway of a finished item according tothe second embodiment.

A full understanding of the structure of the present invention as wellas of the unique fabrication process is thought to be best achieved asthe detailed description of the fabrication process progresses.

In the drawings, FIGURES 1 through 11 illustrate the first embodimentwhich is a liquid-to-liquid heat exchanger, whereas FIGURES 12 throughI7 illustrate a second embodiment according to the present invention.

the emplacement This second embodiment is intended mainly forliquid-togas heat exchanges although it is not limited to thatapplication. The said first embodiment will be fully described beforefurther reference is made to the second embodiment.

Referring now to FIGURES 1 and 2, it will be noted that a multiplesandwich of alternating copper and aluminium sheets is first assembled.The copper sheets illustrated by the numeral I typically are thin sheetsof numher 110 copper alloy in the hardened condition having 266B.t.u./hr./sq. ft./degree Fahrenheit/ft. conductivity. This particularcopper is one which has a very high thermal conductivity and a tensilestrength in mill condition high as 55,000 psi. The aluminium sheets 2are slightly smaller in both flat dimensions than the copper sheets sothat their edges are slightly indented Within the stack as will beobvious from FIGURE 2. The aluminium sheet thickness is chosen inaccordance with the desired fluid flow cross-sectional thickness. Sinceevery other aluminium sheet corresponds to a fluid flow area for one ofthe two fluids passing through the heat exchanger, it is possible foralternate aluminium sheets to be of a first thickness and the remainingones to be of a second thickness. Since fluid flow paths will ultimatelyreplace the aluminium sheets, the result of such a variation inaluminium sheet thickness would be a larger overall flow cross-sectionfor one of the two fluids as compared to the other. Such an expedientmight be desirable, as for example when there is an unlimited coolantsupply and maximum cooling of a hot fluid is sought, or when thespecific heats of the two fluids differed widely. In such a case it maybe desirable to have the rate of flow or the fluid cross-sectional area,or both, greater in connection with the fluid of the lower specificheat. In a particular model of a heat exchanger according to the presentinvention, copper strips 12 inches long by 1 inch wide and .010 inch inthickness were used. Altogether of such copper strips were separated by14 strips of aluminium each slightly smaller in the flat dimensions ascompared to the copper sheets and each .060 inch in thickness, therebyleaving copper sheets on the outside locations. Proceeding now to FIGURE3 the entire assembly is sprayed along the laminated edges of FIGURE 2with a soft aluminium alloy preferably containing little or no alloyingelements. In this way the spaces between the projecting edges of thecopper sheets are filled with aluminium. While this operation isproceeding the sandwich is necessarily held tightly together in sometype of clamping mechanism (not shown). Once the aluminium sprayingoperation of FIGURE 3 is com pleted however, the entire assembly may beunclamped as it will now be held together by the sprayed aluminiummaterial. The appearance of the result of FIGURE 3 is that of arectangular block of sprayed aluminium on two of the sides and the ends,with copper showing on the remaining two opposite faces. The aluminiumspray gun.

3 might be any commercially available metal spray gun loaded to sprayaluminium. The Metco metal spray gun is satisfactory for this operation.

In FIGURE 4 the surface 4 and its opposite number on the bottom of theblock would be the remaining copper surfaces exposed to view after thecompletion of the operation of FIGURE 3. FIGURE 4 illustrates theapproximate appearance of the block after the aluminium sprayed sidesand ends have been machined smooth so that the edges of the coppersheets inside are exposed. The extent of this machining operation is notsuificient to remove any substantial amount of the sprayed aluminiummaterial which was deposited between the projecting edges of the coppersheets. As an additional step in connection with the assembly as itappears in FIGURE 4 a relatively short dip is effected in an aqueoussolution of muriatic acid or hot sodium hydroxide. Either of these twochemicals vigorously attacks the aluminium while leaving the coppersubstantially untouched. Thus the aluminium is etched away from theexposed copper edges. The depth of this etching process has been foundto be on the order of .010 to .030 for an exchanger of the indicatedsize of the model.

As a further intermediate step, the assembly is washed to neutralize theetching chemical, preferably sandblasted to remove any copperdiscoloration or surface corrosion and then washed again and de-greasedin order to prepare it for the plating tank.

It should be noted that in applying the electrode to the assembly forthe plating operation, care should be exercised so that the electrodedoes not grip at or near the exposed copper strip edges.

Referring now to FIGURE 5, the assembly is seen after it has beenelectroplated with a .030 to .040 thickness of copper. It will berealized that the copper plating reaches into the voids created betweenthe copper edges by the etching thereby gripping the copper sheets withthe full strength of the deposited copper material. Referring now toFIGURE 6 the ends of the assembly have been machined into two wedgeshapes. The ends are not machined to a direct point however. A smallstrip of copper typically shown at 6 in FIGURE 6 is left at each end.Obviously this machining operation removes all of the spray depositedaluminium material within the areas of the four cuts made andillustrated. There is still an adequate amount of spray depositedaluminium on the remaining laminated edges now covered by the copperplating, and the assembly may therefore be expected to remain as afirmly joined laminated block.

The thickness of plating to be applied in order to obtain the appearanceof FIGURE 5 will depend upon the overall strength service environmenti.e. the mechanical ruggedness required of the unit as well as theinternally applied pressure contemplated. Referring now to FIG- URE 7and 7a, it Will be noted that the aluminium edges on either end arealternately masked in the manner shown in FIGURE 7a. As either end ofthe block of FIGURE 7 is viewed directly it should resemble FIGURE 7a onone end and the mirror image (complement) of 7a on the other end. Itwill later be seen as the unit is manifolded that the fluid flow is thusfrom 7 to 7 and from 8 to 8 as shown on FIGURE 7. The maskingcontemplated in FIGURE 7 is preferably accomplished through the use ofthin strips of polyethelene film with adhesive, taking care that themasks are tight to the aluminium. Masking can also be done with brushapplied lacquer and removal by solvent. Thus it will be seen from FIGURE7a that the appearance alternates between masking strips and bare stripsof aluminium. The masking strips must not overlap the copper edges sincein FIGURE 8 the entire assembly has again been copper plated and themasks thereafter removed. During this plating step the new copper mustadhere to the exposed copper edges well to prevent internal leaks in thefinal product. The appearance of the end of the assembly contemplated inFIGURE 8 is then one of alternating copper plated aluminium edges andbare aluminium edges.

Referring now to FIGURE 9, four shaped aluminium mandrels are affixed tothe edges of the FIGURE 8 assembly. These mandrels typically at 9 inFIGURE 9 may be fabricated of pure soft aluminium by a process such asdie casting. They may be of thin wall construction but must be closed at10, 11, I2 and at the other manifold mandrel corresponding portion whichis the diagonally opposite number of 9 on FIGURE 9. Aluminized plasteror plastic could be used as an alternative for an aluminium mandrel inthe step of FIGURE 9. A material should not be used which will bedifficult to etch or dissolve away however.

FIGURE 10 represents the appearance of the entire unit after it has beencompletely copper plated with the manifold mandrels in place. The use ofa small amount of adhesive along the edges of the manifold mandrel inorder to hold it in place for the plating operation of FIGURE is notobjectionable provided that in the ultimate step when the unwantedmaterial is dissolved away a suitable solvent can be used to remove thisad hesive. Once the final copper plating is effected, the mandrel endsat 13, 14 and in FIGURE 10 are opened either by drilling or by machiningoh the closed ends to open them. Another dip in the acid or sodiumhydroxide solution is next accomplished. In this way all of thealuminium within the assembly including the original aluminium sheetsand the aluminium manifold mandrel are completely dissolved leaving anall copper assembly as shown in the cutaway view of FIGURE 11. Inaccordance with the masking and plating operation of FIGURES 7 and 8 themanifolds are now automatically joined so as to produce a flow situationdepicted by FIGURE 11. If it is assumed that a hot fluid is representedby the arrows entering at 13 in FIGURE 11 this flow will be dividedamong alternate laminar flow paths and discharged at 14. Theinterleaving flow paths are connected to mainfold 15 and its oppositenumber not shown.

Proceeding now to FIGURE 12 a description will be given of the secondembodiment. In this second embodiment it will be assumed that anexchange of heat is desired between a flowing gas and a flowing liquid.In general the second embodiment operates much like a firetube boiler inthat the gas passes through a plurality of tubes in parallel, and thefluid is circulated around the full length and perimeter of the tubes.

Referring now to FIGURE 12 it may be assumed that a solid block ofaluminium has been drilled through with a plurality of holes of which 21is typical. symmetrically placed saw cuts 19 and have also been made.

Mass production methods for producing the block of FIGURE 12 are readilyavailable and are well known. For the sake of economy of material, it isalso possible to construct the block of FIGURE 12, which actually servesas a mandrel and assembly jig, from two thin prepunched plates, one ofwhich would form the top surface 18 visible in FIGURE 12 and anidentical opposite plate would form the bottom of the block. Thin sheetside members, of which 16 and 17 are typical, could then be welded oreven cemented in place as shown thereby producing a hollow boxstructure.

Proceeding now to FIGURE 13, the slots 19 and 20 each receive a slightlyoversize copper sheet as at and 25 in FIGURE 13. Each of the holes 21now receives a copper tube slightly longer than the thickness of theblock mandrel. The use of a small amount of adhesive cement is entirelyappropriate to hold the tubes 22. and the plates 25 and 26 in placepending the next operation, although tight fits in the holes and sawcuts could be depended upon to hold these parts in place. Hollowcylindrical copper intake and exhaust conduits 23 and 24 will requiresome adhesive to hold them in place however. The end product of the stepof FIGURE 13 is then the aluminium mandrel and jig block of FIGURE 12with all of the holes 21 filled by copper tubes 22 and copper plates andconduits 25, 2623, 24 respectively in place as shown.

In FIGURE 14 the assembly of FIGURE 13 is aluminium sprayed in a stepwhich is a counterpart of FIG- URE 3in connection with the firstembodiment.

FIGURE 15 envisions a machining step of the two surfaces through whichthe smaller copper tubes are exposed and is comparable to the step ofFIGURE 4.

In FIGURE 16 the entire assembly is electroplated with copper in a stepcomparable to FIGURE 5. It is assumed that the mouth of each of thecopper tubes including the larger conduits 23 and 24 have been suitablyplugged with a material to which the plating will not adhere. It shouldalso be noted that immediately prior to plating the same preparationsteps as in connection with FIGURES 4 and 5 are assumed, including theshort etching step whereby the edges of the copper tubes are leftprojecting by a very small amount above the aluminium surface 29 beforethe copper plating is applied. In this way the plating which isdeposited over the surface 29 as well as 30, 31 and for that matter over23 and 24 will effect a strong union with each of the tubes. After thisplating operation, the assembly is substantially finished except for thedissolving of the aluminium block mandrel by the same method asdescribed in connection with the first embodiment.

FIGURE 17 illustrates the manner in which the liquid and gas might beexpected to flow. Liquid admitted at 23 would flow past the first thirdof the copper tubes and would be constrained from lateral flow by thebaffle 34. It would then round the corner of the baffle 35 and flowbetween the baffles 34 and 33 until it again reversed direction andflowed over the remaining third of the tubes between 33 and sidewall 30.A typical outside tube 32 would be expected to be emplaced so as toclear a side wall opposite 3G by approximately the same amount as itclears any adjacent companion tube.

The fundamental similarity between the process of forming the twoembodiments will readily be noted in the machining operation of FIGURE15. For example, it is expected that not all of the sprayed aluminium isto be removed from the machined surfaces for the same reasons asdiscussed in connection with FIGURE 4. It is assumed also that whatevermaterial is used as a nonconductive plug such as cork, rubber or plasticfor temporarily blocking the tubes 22 remains substantially in placeduring the machining step of FIGURE 15 and the plating step of FIGURE 16but is removed before the application of the aluminium dissolvingchemical.

From the foregoing description it will be obvious to one skilled in theart that various modifications and variations are possible and can bepracticed without further invention. For example, the original metalsheets can be shaped other than rectangularly. They may even assume acomplex curved shape or some other form factor.

It will also be understood that in the first embodiment the surfacesfrom which the intake and discharge ports (as for example in FIGURE 10)13, 14 and 15 are emplaced could be corrugated for strength in adirection such that ridges of the corrugations run perpendicular to thelong dimension of the unit. Each of the 1 and 2 sheets (FIGURE 1) wouldin that event be individually corrugated. In this way additionalstrength against internal pressure could be achieved. Corrugation of theside surfaces of the typical second embodiment such as at surface 30could also be effected. The corrugations in that case would also beexpected to have their ridges normal to the long dimension of side 30.The wedge shaped cuts of FIGURE 6 are convenient, but no critical anglesor cut widths are suggested by the illustration.

Obviously the ports or conduits 23 and 24 as of FIG- URE 16 could bemasked or coated prior to the plating operation to avoid excessivebuildup of plating material thereon if so desired.

Concerning materials, virtually any plateable material can be used inlieu of the copper and another metal could replace the aluminiuminterleaves, spray metal and manifold mandrels. The etching chemicalmust be such that it would dissolve the aluminium substitute withoutsubstantially attacking the copper substitute. Many non-ferrous metalsincluding the noble metals could be chosen to provide suitable metalpairs and a general knowledge of chemistry would permit the properchoice of an etching chemical.

Many additional possibilities will suggest themselves in respect tomaterials used. Among other things, whenever the copper plating ofeither embodiment is to be undertaken, a thin plating of gold rhodium orsome other selected plateable metal of exceptional nobility could firstbe applied. If the copper plating already discussed is then applied overthe noble metal flashing or plating, fluid contact surfaces could thendisplay the advantage of the noble metal coating where corrosive fluidsare involved.

The copper sheets in the stack of the first embodiment and all thecopper tubes of the second embodiment would need to be preplated orflashed with the noble metal to be used in order to complete theinterior surface of noble metal in the finished product.

One of the most important results of the present invention was thedevelopment of the unique process described which permits the retentionof mill hardness and strength in the copper (or other material) whichforms the final product. Heat, as has been pointed out before,deteriorates these qualities of the metal and therefore the device ofthe present invention is thereby greatly superior to any design callingfor the use of heat to weld, braze, or even solder. The retention ofmaximum metal strength than allows minimum metal thickness andcorrespondingly more efficient heat transfer. Bi-metal or galvaniccorrosion Within the present invention is not a factor because the metalseen by the fluids is all the same element.

While it has been contemplated that copper plates and tubes used in thefabrication of the embodiments of the present invention would normallybe desired to be in their fully work hardened and most thermallyconductive condition, this is not a requirement of the process. If, forsome special purpose softer alloys were desired, the process will befound to be fully applicable.

In presenting the present invention with drawings depicting typicalshapes and configurations, it is pointed out that the inventor does notthereby wish to be limited to the scope of his invention. The drawingsare illustrative only and in accordance with the broad concepts of theinvention, what is claimed is:

1. The process for producing a multi-chambered fluid tight vesselincluding at least two independent fluid flow chambers in close thermalcontact with each other comprising: embedding a plurality of thin wallelements of a first metal in a mass of a second metal to the extent thatonly the extremities of said elements protrude from said mass of secondmetal, said second metal being capable of being selectively etched andremoved from said first metal; masking a plurality of areas about theperiphery of said mass with a material to which electroplating will notadhere, said masking being applied so as to correspond to areas ofaccessibility to said fluid flow chambers; electroplating said mass withmetal substantially the same as said first metal; removing said maskingmaterial thereby to provide access openings; and bathing said platedmass in a chemical agent which does not appreciably react with saidfirst metal but which reacts with said second metal to completelydissolve all of said second metal.

2. The invention set forth in claim 1 in which said first metal iscopper and said second metal is aluminum.

3. The method of constructing a fiuid-to-fluid heat exchanger comprisingthe steps of: stacking at least three sheets of a first metal separatedfrom each other by interleaving sheets of a second metal, said sheets ofsecond metal being slightly smaller in the flat dimensions than saidsheets of first metal, said second metal being one capable of beingselectively etched and removed from said first metal; spraying more ofsaid second metal as a binder into the indentations produced around theedges of said sheets of second metal, thereby to produce a cohesiveassembly; machining said sprayed portions Hat and to an extent toexhibit flush edges of both of said metals and etching said second metalslightly away; plating said assembly with a continuous layer of saidfirst metal; machining at least one surface perpendicular to said sheetson each of two opposite faces of said assembly, thereby exposing thecross-sections of all of said sheets of first and second materials;alternately masking at least some of said exposed sheets of said secondmetal, replating said assembly with said first metal and thereafterremoving said masking; manifolding said machined surfaces and dissolvingall of said second metal from said assembly with a chemical whichattacks said second metal but not said first metal.

4. The process for producing a device of the character describedcomposing the steps, in order, of: arranging and separating a pluralityof congruent relatively thin wall members of a .plateable first metal bymeans of separator members of a second metal, said second metal beingcapable of being selectively etched and thereby removed from said firstmetal; spraying an additional quantity of said second metal over saidthin wall members and said separator members to produce a firmly heldmass; machining said mass sufficiently to expose the extremities of saidthin wall members flush with the surface of said sprayed second metal;etching said machined mass with a chemical agent which dissolves saidsecond metal but not said first metal to produce the effect of indentingthe residue of said sprayed second metal away from the edges of saidfirst metal; masking a predetermined area of the exposed surface of saidresidue of sprayed second metal with a masking material to whichelectroplated material will not adhere; electroplating said mass with athird metal, said third metal being of a type which is not etched bysaid chemical agent and which is capable of bonding to said first metalduring said electroplating; removing said masking material; anddissolving all of said remaining second metal by a second application ofsaid chemical agent.

5. The invention set forth in claim 4 in which said manifolding isaccomplished by affixing a hollow manifold mandrel of said second metalto said machined surfaces and again plating said assembly with saidfirst metal.

6. The invention set forth in claim 4 in which said machined surfacesare two in number on each of two opposite ends of said assembly, andsaid masking is accomplished so as to produce two sets of interleavedthrough passages when said second metal is dissolved away.

7. The invention set forth in claim 4 further defined in that said firstand third metals are substantially the same material.

8. The invention set forth in claim 4 further defined in that an initialstep is included of plating said thin wall members with a thin layer ofa fourth metal chemically less active than said first metal, and anintervening step of plating said mass with said fourth metal is alsoincluded before said step of plating with said third metal.

9. The invention set forth in claim 4, further defined in that saidfirst and third metals are metals having a high order of heatconductivity such as copper and said second metal is aluminum containingno substantial quantities of alloying elements.

10. The method invention set forth in claim 9 in which said chemicalagent is a hot alkaline compound such as sodium hydroxide.

11. The method invention set forth in claim 9 in which said chemical isan acid such as muriatic acid.

References Cited UNITED STATES PATENTS 2,587,252 2/1952 Van Weenen etal. 29157.3 2,837,817 6/1958 Kelly 29424 2,855,664 10/1958 Griifith etal 29424 3,044,160 7/1962 Taffee 29-423 3,063,142 11/1962 Kroan 294243,153,278 10/1964 Martin et al 29-424 THOMAS H. EAGER, Primary Examiner.

H. D. HOBART, Assistant Examiner,

